1. Field of the Invention
The instant invention relates to the surface treatment of plastic articles and, more particularly, relates to an improved apparatus for treating the surface of plastic articles so that the surface has enhanced printing and adhesion characteristics.
2. Description of the Prior Art
In the use of plastic materials for packaging, containers, tubular structures, medical products and a variety of other products and uses, it is desirable to print on the surface of these plastics. The most common approach for printing on the surface of plastics, such as polyethylene, involves printing inks. These inks, however, frequently have not satisfactorily adhered to the untreated surface of plastic articles onto which printing is desired. As a result, often not only was the quality of the printing poor, but the ink often could be readily rubbed off the surface of the article.
It has been discovered that treating the surface of the plastic would enhance printability and adhesion of the printing inks thereon, and that the use of a corona discharge treatment method was found to produce suitable results. To achieve such surface treatment by the corona discharge approach, the plastic material is introduced into a region of ionized air in a gap between two charged electrodes. High energy articles bombard the surface of the plastic material in the ionized region. Covalent bonds on the surface of the material are broken as a result of the particle bombardment energies developed in the ionized region. Free radicals that are formed on the surface rearrange to form functional groups which directly affect surface wetting and adhesion. Molecular transformations and dissociations may form species with a chemically altered surface favorable for adhesion. Increased adhesion is the result of an electrical formation induced in the polymeric material by the corona discharge. In short, the particle bombardment on the surface of the plastic material creates microfissures, increased porosity and polar groups which improve adhesion and surface wettability characteristics.
It is well known to treat the surface of polymeric films to enhance the wettability thereof using the corona discharge method. British Patent No. 765,545 discloses such a process in which the surface of a polyethylene or other plastic film is treated to rendor it receptive to receive printing ink and the like. U.S. Pat. No. 4,392,178 describes an apparatus for rapidly enhancing the piezoelectric properties of polar polymeric films by continuous corona poling thereof.
Others have conducted research or testing to correllate the effects of corona treatment and the increase in adhesion characteristics: Baum, et al., "Further Observations On The Decay of Surface Potential of Corona Charged Polyethylene Films," J. Phys. D., Appl. Phys., Vol. 10, 1977, pp. 2525-2531; Carley, et al., "Corona-Discharge Treatment of Polyethylene Films. 1." Polymer Engineering and Science, March, 1978, Vol. 18, No. 4, pp. 326-334; Stadal, et al., "The Effect of Corona and Ozone Treatment on the Adhesion of Ink to the Surface of Polyethylene," Polymer Engineering and Science, January, 1977, Vol. 17, No. 1, pp. 38-41; and Ehrbar, et al., "Using Corona Discharge to Deglaze Plastic Films Before Metalizing," Plating and Surface Finishing, February 1980, pp. 64-66.
Treating the surface of formed plastic articles using a continuous corona discharge in an assembly line setting is disclosed in U.S. Pat. No. 4,724,508 to Macy wherein formed plastic work pieces are passed between a series of aligned, opposed electrode pairs, an air flow passed through the volume between said pairs of electrodes and the surface of said plastic articles thusly treated. A number of shortcomings, however, have been noted in the Macy system. Among them, firstly, is that the air flow in Macy is directed in such a way as to allow the escape of dangerous ozone into the atmosphere. Secondly, the direct alignment of the electrode pairs results in a corona field which yields less than 360.degree. of uniform coverage over the surface of the work pieces. The use of directly opposed, as opposed to staggered, electrode positioning yields high and low intensity treatment areas on the work pieces due to the non-uniform geometry of some work pieces and the fact that the surfaces of the work pieces closest to the electrodes tend to become low intensity treatment areas as the transverse air flow passes along a line passing between electrode pairs. Thirdly, Macy uses a plurality of air flow manifolds which are placed side by side, allowing dangerous corona leakage from the interfaces of the respective manifolds.
It is also desirable to control the temperature of the air flow so as to control the temperature of the electrode and to control the heat generated by the electrodes to avoid failure of the dielectric electrode covering. Such failure creates a non-uniform corona field with hot spots which ultimately causes an electric arc.